What is spam?
How spammers get your email address
How to spot spam
Filtering spam in Microsoft Outlook 2003 and 2007
Filtering spam in Outlook Express and other email clients
Additional tips for dealing with unwanted email
The impact of spam on the environment

What is spam?
Unwanted email, sometimes known as junk or spam email, is unsolicited email advertising.

Use of the word Spam comes from a Monty Python sketch in which a couple attempt to order breakfast at a restaurant where all the menu items feature Spam. In the background a group of vikings sing about Spam and, eventually, any hope of meaningful communication is lost in the drone of "Spam." Sound familiar?

Examples of unwanted email are:

• Adverts for porn or gambling sites.
• Work from home schemes.
• Online pharmacies.
• Advertising “Herbal Viagra” and similarly improbable products.
• Selling implausibly cheap (pirated) software.

A substantial majority of the email that goes over the internet every day is unwanted junk mail.

• Guessing. Spammers use automated software to generate addresses.
• Harvesting from websites. If an email address is embedded in a webpage, spammers can use spiders (similar to those used by search engines) to find them.
• Online registration. If someone gives their email address to a dodgy website or one with a lax privacy policy, the site owner can sell the addresses to spammers.
• From other spammers. It's possible to buy lists of email addresses by the million online.
• So-called spam email cancellation services. These bogus services offer to block unwanted email but really collect addresses.

• From someone you don't know.
• Contains weird misspellings (e.g. “p0rn” with a zero), designed to counter spam filters.
• Offers something for sale.
• The subject line and contents don't match.
• Contains a beguiling or urgent call to action (“Buy now to get 50% off”).

If you get more than a handful of unwanted emails a day it is possible to use a mail filter to get rid of the rubbish automatically.

Microsoft Outlook Express and other email clients
Outlook Express does not have a built-in filter but there are a couple of options you can try:

• Switch email clients. Mozilla Thunderbird is a free, open source email client that does almost everything that Outlook Express does (the main exception is Hotmail support) but adds additional features such as a spam filter. 
• Buy or download a free filter program.

These are the same options as for older versions of Microsoft Outlook. If you use a different mail client, see if it has a filter option or if you can upgrade it to a newer version which does. Alternatively, consider switching to a webmail or email client with effective spam filtering features.

Additional tips for dealing with unwanted email
Pick long or obscure email addresses, with a combination of letters, numbers and punctuation marks to make it harder for spammers to guess.

• If you use a spam filter, report spam that gets through the system so that the filter can catch it in future.
• Check junk mail folders regularly in case a legitimate email gets through by mistake.
• Never click on links in spam.
• Never open attachments in spam.
• Never buy things or make charitable donations in response to spam email. If people stop buying, the spammers will stop sending.
• Never click on “unsubscribe” or reply to unwanted email.
• There are no spam email opt-out lists. Anyone advertising one is trying to get your email address or money.
• Only give your real, primary email address to people you know.
• Don't list your main email address in internet directories or other public sites.
• Set up a throw-away email address for online commerce and site registration using a free webmail service.

The impact of spam on the environment
An estimated worldwide total of 62 trillion spam emails were sent in 2008

• Globally, annual spam energy use totals 33 billion kilowatt-hours (KWh), or 33 terawatt hours (TWh). That’s equivalent to the electricity used in
  2.4 million homes in the United States, with the same emissions as 3.1 million passenger cars using two billion United States gallons of gasoline
• Spam filtering saves 135 TWh of electricity per year. That’s like taking 13 million cars off the road
• If every inbox were protected by a state-of-the-art spam filter, organizations and individuals could reduce today’s spam energy by approximately 75 percent or 25 TWh per year. That’s equivalent
  to taking 2.3 million cars off the road
• The average emission associated with a single spam message is 0.3 grams of CO2. That’s like driving three feet (one meter) in equivalent
  emissions, but when multiplied by the annual volume of spam, it’s like driving around the Earth 1.6 million times. A year’s email at a typical medium-size business uses 50,000 KWh; more than one fifth of that
  annual use can be associated with spam
• Filtering spam is beneficial, but fighting spam at the source is even better. When McColo, a major source of online spam, was taken offline
  in late 2008, the energy saved in the ensuing lull — before spammers rebuilt their sending capacity — equated to taking 2.2 million cars
  off the road
• Much of the energy consumption associated with spam (52 percent) comes from end-users deleting spam and searching for legitimate email (false positives). Spam filtering accounts for just 16 percent of spam-related energy use.